Clock generator circuits are well known circuits that are used in a myriad of applications. In modern integrated wireless/wireline receivers, transmitters and transceivers, as well as digital processors and system-on-chip (SoC) circuits, clock generator circuits are an essential block. In particular, quadrature clock generator circuits are often used to generate multiple clock signals 90 degrees apart from each other. To reach a very high data rate in clock and data recovery integrated circuits, a multiphase oscillator is typically used. A common way to generate quadrature clock signals, is to use a conventional ring oscillator. In ring oscillators, oscillation is made based on inverter delay with capacitive and/or resistive load. A drawback with these types of oscillators though is that the phase noise is relatively high compared to LC type of oscillators.
A circuit diagram illustrating an example prior art LC type oscillator with quadrature outputs is shown in FIG. 1. The oscillator, generally referenced 10, comprises two cross coupled LC oscillators, namely oscillator A and oscillator B. The first oscillator is coupled in-phase to the second, but the second one is coupled anti-phase to the first. MOS transistors are used as the coupling devices. This idea can be extended by coupling a higher number of oscillators as shown in FIG. 2. A circuit diagram illustrating an example prior art LC type oscillator with an additional number of oscillators is shown in FIG. 2. The structure, generally referenced 20, allows an additional number of different phases to be generated.
In the oscillators of FIGS. 1 and 2, coupling the oscillators forces the oscillation frequency to be shifted away from the resonant frequency of each LC tank. Therefore, each tank shows a lower quality factor Q. In this way, quadrature accuracy can be traded-off with phase noise. The higher the coupling strength the better the in-phase/quadrature (IQ) phase accuracy, but the worse the oscillation phase noise.
A circuit diagram illustrating an example prior art multiphase LC type ring oscillator is shown in FIG. 3A. A circuit diagram illustrating an example implementation of each stage of the oscillator of FIG. 3A in more detail is shown in FIG. 3B. FIG. 3A shows a multiphase LC ring oscillator, generally referenced 30, that utilizes a parallel LC tank instead of a resistive/capacitive load as used in prior art conventional ring oscillators. The circuit, generally referenced 40, for each stage of oscillator 30 is shown in FIG. 3B.
Since it has a much higher open loop Q than the conventional ring oscillator, it has substantially better phase noise performance for the same current consumption. Compared to the oscillator of FIG. 1, cross-coupled pairs are eliminated in the structure of FIG. 3A to avoid potential parasitic oscillation modes. In this oscillator structure, oscillation frequency is still different than the resonant frequency of each LC tank. Therefore, each LC tank shows a lower quality factor resulting in a worse phase noise. In the above three types of prior art oscillators (i.e. FIGS. 1, 2, and 3A), active coupling devices inject a current into an LC tank that is not in-phase with the voltage of the tank. Consequently, noise from the coupling device is converted to phase noise with a higher gain, compared to the case where injected current is in-phase with the voltage of the tank.
A circuit diagram illustrating an example prior art rotary traveling-wave oscillator (RTWO) is shown in FIG. 4. The structure of the RTWO, generally referenced 50, is the rotary ring composed of a differential transmission line and several active devices to compensate losses. As a wave starts to propagate around the loop, it travels 360 degrees in each rotation cycle. The use of high quality passive resonators (especially the inductors and transmission lines) results in better phase noise performance compared to a conventional ring oscillator. As transistors of the inverters enter the triode region, however, the total quality factor Q of the resonator decreases and consequently degrades phase noise performance. Note that an alternative version of the RTWO can be implemented using lumped components.
A circuit diagram illustrating an example prior art multiphase LC type ring oscillator is shown in FIG. 5A. A circuit diagram illustrating an example implementation of a negative Gm cell stage of the oscillator of FIG. 5A in more detail is shown in FIG. 5B.
In this implementation, the oscillator, generally referenced 60, comprises negative Gm elements that are each implemented by a pair of current biased inverters shown in circuit 70 in FIG. 5B. In this manner, the inverters do not load the quality factor Q of the tank. The resonant frequency of the oscillator of FIG. 5A is less than the resonant frequency of a single LC tank, making it less attractive for high frequency applications.
A circuit diagram illustrating an example prior art multiphase ring oscillator using a CL ladder configuration is shown in FIG. 6. The oscillator, generally referenced 80, is constructed as a multiphase oscillator. This oscillator uses a CL ladder rather than an LC ladder by swapping the positions of L and C. It can be shown that in an eight phase oscillator, the CL ladder configuration provides a higher resonant frequency than a LC ladder. In addition, the oscillation frequency of this oscillator is higher than the resonant frequency of a single CL tank. The oscillation frequency, however, cannot go above the resonant frequency of the inductor and parasitic parallel capacitance of the active devices.
A circuit diagram illustrating an example prior art LC delay line oscillator is shown in FIG. 7A. A circuit diagram illustrating an example implementation of a gain stage of the oscillator of FIG. 7A in more detail is shown in FIG. 7B. The oscillator structure, generally referenced 90, comprises an LC delay line oscillator configuration. In this structure, four LC delay lines are used in a loop, with each delay line driven by a gain stage 100 shown in FIG. 7B.
In this circuit, the output load resistance of the gain stage matches the characteristic impedance of the delay line. In this oscillator, most of delay around the oscillator loop is provided by the delay line, making the VCO resistant to variations in power supply, temperature and process. The oscillator frequency is the same as the resonant frequency of the LC delay line. The use of an LC tank improves the phase noise of this structure compared to conventional ring oscillators. The use of resistively loaded gain stages, however, introduces extra loss in the oscillator, making it less power and phase noise efficient.
There is thus a need for an oscillator that overcomes the disadvantages of the prior art oscillator circuits. Preferably, the oscillator is able to generate quadrature phase output signals, is simple and accurate, exhibits low phase noise, has a low cost of manufacturing, is power efficient and consumes relatively little semiconductor real estate.